The present invention relates to the fabrication of semiconductor devices.
Plasma-etching processes are commonly used in the fabrication of semiconductor devices. Generally, photoresist material forms feature patterns on the surface of the wafer to be etched, and features are then etched into the wafer by exposing the wafer to a particular type of etching gas. One of the challenges faced in plasma etching is the ever-increasing aspect ratio needed to meet design requirements, especially for ultra-high density structures. When etching features on semiconductor wafers, the aspect ratio of an etched feature is defined as the ratio between the feature's depth (d) and the feature's width (w) or diameter. As more features are packed on a single piece of wafer to create higher density structures, the width (w) or diameter of each individual feature necessarily decreases, while the depth of the features remains unchanged. Thus, the aspect ratio of each individual feature increases as the device feature shrinks.
A new difficulty emerged recently during ultra-high aspect ratio (UHAR) etch is twisting, which is generally defined as deviations of location, orientation, shape, and size near the bottom of a feature from the pattern defined by the mask on the top of the feature. When the aspect ratio of a feature reaches a certain threshold while the feature's width is very small, twisting occurs, particularly near the bottom of the feature.
FIG. 1A shows the side view of a wafer with four etched features, where twisting occurred near the bottom of each feature. In FIG. 1A, a mask 100 deposited on the surface of the wafer 110 masked four features 120, 122, 124, 126. The four features 120, 122, 124, 126 are etched into the wafer 110 all the way to the stop layer 130. The four features 120, 122, 124, 126 each has a width, W, and a depth, D. The aspect ratio of each of the four features 120, 122, 124, 126 is D/W. As shown in FIG. 1A, the sidewalls of the features 120, 122, 124, 126 are not straight but are curved near the bottom of the features. Such deviations near the bottom of the features 120, 122, 124, 126 from the pattern defined by the mask 100 are twisting.
Generally, twisting may manifest in one or more of the following forms. Lateral twisting is defined as deviation of the etch profile center from the vertical straight line near the bottom of a feature. Angular twisting is defined as deviation of the angular orientation of the etch profile from a predefined angle near the bottom of a feature. Shape twisting is defined as deviation of the etch profile from a predefined uniform shape near the bottom of a feature. All three forms of twisting may occur simultaneously within a single feature.
FIG. 1B shows the top view of samples of various forms of twisting near the bottom of the etched features. When twisting occurs, the oval-shaped cross-section is not perfect in size, position, orientation, shape, or a combination thereof. In FIG. 1B, for lateral twisting, the cross-sections 140, 142, 144, 146, 148 shift so that the center of the ovals are no longer at the center of the original pattern. For angular twisting, the cross-sections 150, 152, 154, 156, 158 rotate so that the ovals are no longer aligned with the original pattern. For shape twisting, the cross-sections 160, 162, 164, 166, 168 are no longer in the original oval shape. Finally, in case of combination twisting, the cross-sections 170, 172, 174, 176, 178 simultaneously shift, rotate, and change shape. All forms of twisting result in defects and device failures. Therefore, it is desirable to reduce or eliminate twisting during UHAR etch.